The present invention relates to the preparation of spheroidal polymer beads, particularly to a method and apparatus for preparing spheroidal polymer beads having a relatively uniform particle size.
A wide variety of polymers including polymers of a monovinylidene aromatic such as styrene or esters of an unsaturated carboxylic acid such as acrylic or methacrylic acid esters are prepared in spheroidal bead form using suspension polymerization techniques. Conventionally, suspension polymerization consists of dispersing a monomer as droplets in a vessel equipped with an agitator and containing a continuous phase in which the monomers and resulting polymer are essentially insoluble. The dispersed monomer droplets are subsequently polymerized while maintaining continuous agitation.
Unfortunately, the spheroidal polymer beads prepared by conventional suspension polymerization techniques exhibit a relatively broad particle size distribution. This is generally undesirable for the reason that spheroidal polymer beads employed in most applications advantageously exhibit a generally uniform particle size. For example, the ion exchange resins prepared from copolymer beads of a monovinylidene aromatic and a polyvinylidene aromatic advantageously exhibit a narrow particle size distribution to provide the desired flow characteristics to an ion exchange resin bed comprising said resin. Therefore, a significant percentage of the polymer beads prepared using conventional suspension polymerization techniques possess particle sizes undesirable for their intended application. Such undesirably sized beads are normally wasted or employed in applications of less economic desirability.
Heretofore, various suspension stabilizers (suspending agents) have been employed to help improve the uniformity of the polymer beads prepared using conventional suspension polymerization techniques. (See, for example, U.S. Pat. Nos. 3,728,318 and 3,862,924.) While the suspending agents effectively maintain the monomer droplets in suspension, they do not substantially reduce the broad particle size distribution obtained when the monomer is suspended in the continuous polymerization medium.
Alternatively, various modifications in the suspension polymerization process have been proposed to prepare more uniformly sized polymer beads. For example, U.S. Pat. No. 2,694,700, in describing a method for the continuous suspension polymerization of vinyl compounds, teaches the coalescence of the suspended monomer droplets can be reduced by introducing substantial amounts of the polymer being prepared to the monomer prior to its suspension in the polymerization medium. Although effective in reducing coalescence of the suspended droplets, an undesirably broad particle size distribution is obtained in suspending the monomer as droplets in the polymerization medium, thereby causing the resulting polymer beads to possess a relatively broad particle size distribution.
U.S. Pat. No. 2,932,629 reports that polymer beads of a relatively uniform size can be formed by dispersing the polymerizable monomers as droplets having a smaller size than the desired size of the polymer beads to be prepared therefrom and allowing the limited coalescence of the dispersed monomer droplets until they reach the desired size. The resulting suspension is then stabilized and the monomers polymerized to form polymer beads. Unfortunately, the disclosed method requires careful control, particularly during the limited coalescence of the monomer droplets, to obtain polymer beads having desired particle sizes.
In yet another method for preparing uniformly sized beads, U.S. Pat. No. 3,922,255 proposes forming monomer droplets by jetting a mixture of a polymerizable monomer and a polymerization initiator through capillary openings into a column of a continuous aqueous phase containing a suspension stabilizer and partially polymerizing the thus formed monomer droplets. The polymerization of the partially polymerized droplets is subsequently completed in a reaction vessel which is specifically designed and equipped with an agitation means for the polymerization of the previously suspended monomer droplets. While the uniformity of the suspended monomer droplets can be increased using the proposed method, the particle size distribution is often sufficiently broad that an undesirably large portion of the final beads are wasted or must be employed in less economically desired applications. In addition, this process requires bulky and expensive equipment, is essentially a continuous process, and due to slow droplet formation, does not provide an economical means for preparing a suspension high in organic content such as is necessary for commercial production of copolymer beads.
In view of the stated deficiencies of the prior art methods for preparing uniformly sized polymer beads, it remains highly desirable to economically and efficiently prepare spheroidal polymer beads having improved uniformity.